Frozen foodstuffs, because of permitting a long-term preservance and transportation between mutually distant food producing and consuming locations, have now taken an increasingly important part in the food industry and markets. The following table illustrates typical frozen foodstuffs in commerce and their respective storable periods with respect to the freezing temperatures used.
______________________________________ Foodstuff Freez. Temp. Storage (months) ______________________________________ Beef -18 (.degree. C) 8 - 12 -24 18 Pork -18 4 - 6 -24 8 - 10 Mutton -20 6 - 10 Fowl -18 3 - 4 Fish (fatty) -18 2 - 3 Fish (less fatty) -18 4 - 6 Shellfish -18 3 - 4 Cooked fish -18 3 - 4 Soup -18 4 - 6 ______________________________________
Techniques which have been employed heretofore to freeze and defrost foods, however, are not at all satisfactory to provide a recovered food which is indeed comparable in quality with fresh or unfrozen one. For example, "fast thawing" by rapidly heating at an elevated temperature a frozen meat may ruin the texture thereof and, if undesired "cooking" can be avoided, causes a considerable dripping of natural meat juice or moisture to the extent that the meat as defrosted entirely looses its natural flexibility, color, appearance and flavor. "Natural thawing" in which a frozen food is placed in an oven or refrigerator at a moderate temperature can alleviate this problem but this process as practiced in the manner which has been done in the prior art is extremely time-consuming and thus not entirely satisfactory on the practical basis. Yet, foods defrosted by conventional natural thawing leave much to be desired in the resulting freshness, taste and appearance. Moreover, exposure of perishable foods to a defrosting temperature for a lengthy time is undesirable because of danger of spoilage of the outer layer while the inner body is still being defrosted. An additional problem is that lengthy exposure of practically all fresh foods to the atmosphere gives rise to development of exidation and renders the defrosted food discolored or unappetizing.
It is also significant to note that there has been no successful method proposed and practiced heretofore of preserving or freezing and defrosting fresh vegetables and fruits so as to substantially maintain their unfrozen freshness. It is said that a leaf vegetable such as spinach permits a storage for 40 days if frozen and maintained at -10.degree. C and a storage for as long as 300 days if frozen at -20.degree. C. These frozen foods when defrosted naturally, however, become pasty, loosing their original appearance substantially entirely, and such a tendency is found more or less with practically any kind of fresh vegetables and fruits which have been frozen. The conventional freezing and defrosting techniques can, therefore, be applied only to those vegetables which may be cooked simultaneously with defrosting and are totally impractical or unsatisfactory for those vegetables and fruits which are most often to be desirably served without cooking or boiling. A procedure which has consequently been used in the prior art is to keep them at a temperature slightly higher than their respective freezing point. In the range of these temperatures, i.e. -1.degree. to 4.degree. C, however, bacteria and micro-organisms tend to propagate relatively actively and reduction of vitamin content as an important constituent of vegetables, takes place markedly so that with the lapse of time they if not completely spoiled become unavoidably inferior both in taste and appearance.